Calorized steel article



Jan. 15, 1935. B. J.. SAYLES 1,988,217

CALORIZED STEEL ARTICLE Filed June 15, 1934 2 Sheets-Sheet l .040 INCHESZ 'IIIIIII/II/IIIIIIIIIII/I/ IIIII/IIIIfi/II/IIIII B. J. SAYLESCALORIZED STEEL ARTICLE Jan. 15, 1935.

Filed June 15, 1934 2 Sheets-Sheet 2 .025 INCHES lNVENTOR Patented Jan.15, 1935 UNITED STATES VPATENT OFFICE CALOBIZED STEEL ARTICLE Bertram J.Sayles, Pittsburgh, Pa.

Application June 15, 1934, Serial No. 730,737

17 Claims. (01. 14831) The present invention relates to calorized steelarticles, and more especially to articles intended for use attemperatures normally not in excess of about l500 Fahrenheit, and whichmay be subject to deformation or abrasion.

Examples of such articles are oil still tubes, superheater tubes,recuperator or air heater tubes, heat exchanger and condenser tubes,automobile exhaust piping, and various other articles which require aresistance to oxidation and corrosion, and which require a tightlyadherent ductile surface.

For example, oil still tubes are operated at wall temperatures of about1000 to 1300" Fahrenheit and are subjected to high pressures at thesetemperatures. The outside of the tubes is exposed to the oxidizingaction of the heatin gases and the inside of the tubes is subjected tothe corrosive action of sulphurous compounds in the oil. Carbon isdeposited on the inside of the tubes, requiring frequent cleaning of thetubes by tube cleaners to cut out the tightly adherent carbon deposits.In installing the tubes in the oil still the ends of the, tubes areexpanded to fit the end fittings, so that the tubes and anyrprotectingcoatings on the tubes must be sufliciently ductile to withstand suchexpansion.

Superheater tubes for steam boilers are subjected to somewhat similarconditions, in that they are exposed to the oxidizing and corrosiveeffect of the furnace gases. In fabricating superheater tubes they aregenerally bent, which requires that any protecting coating must be ofsufllcient ductility to allow the tubes to be bent without cracking orspalling off. The interior of the tubes is also subject to oxidation dueto the dissociation of the steam.

' In the case of the exhaust piping for an automobile, the steel issubjected to both oxidation at high temperatures and to acid corrosiveattack. The pipe between the exhaust manifold and muflier becomes heatedso that its outside is subjected to destructive oxidation. The inside ofthe pipe is also subjected to the corrosive attack of the highly heatedexhaust gases. The tail pipe beyond the muflier is subjected to acidcorrosive attack by the condensate from the exhaust gases. Theautomobile piping is usually bent cold, which requires that the pipingand any protective coating thereon should be able to withstand such coldbending.

As can be seen from these specific examples, the steel must have a highsurface stability, that is to say, it must be highly resistant to changeor deterioration such as produced in ordinary steel for example byoxidizing, corroding, or the like, agents. Such surface can be impartedby the well-known calorizing process. The aluminum coating as heretoforeapplied by the ordinary calorizing processes has not been ductile andtends to exfoliate or crack off when the surface is subjected todeformation or abrasion. The calorizing coating usually contains about50% or more aluminum concentration. While it has a high surfacestability in resisting oxidation and corrosion, this high aluminumsurface alloyage is extremely brittle and cannot be deformed withoutrupture and spalling from the steel beneath. It is not tightly adherentto the steel, since there is a fairly sharp line of demarcation betweenthe high concentration of aluminum on the surface and the body of thesteel.

These characteristics have precluded the successful application ofcalorizing to articles which are subject to deformation or abrasion.

I have discovered that the usual calorizing coating may be modified soas to impart to it a considerable ductility and great adherence to thebody of the steel, and at the same time retain a sufficiently highresistance to oxidation and corrosion for the purposes intended. Brieflystated, I have found that by a proper heat treatment following the usualcalorizing operation, the brittle high aluminum concentration at thesurface may be diffused into the body-of the steel without destroyingits high surface stability but imparting to it the necessary ductilityand adherence to the underlying metal required in articles which aresubject to deformation or abrasion.

' In the drawings, which illustrate certain pre-; ferred embodiments ofmy invention,-

Figures land 2 are diagrams illustrating the diffusionof the aluminuminto the steel in accordance with my invention;

Figure 3 is a view, partly in section, of two oil still tubes embodyingmy invention and a standard return bend fitting into which they areexpanded;

Figure/i is a similar view showing the tubes as expanded to fit anothertype of return bend fitting;

Figure 5 is a section through one of the tubes showing the aluminizedsurfaces;

Figure 6 is an elevation of the piece of automobile exhaust pipingbetween the manifold and Figure 7 is a similar view of a tail pipe;

Figure 8 is a section through the piping showing the aluminized surface;and 7 Figures 9 and 10 are an elevation and a partial plan view,respectively, of a superheater tube embodying my invention illustratingthe bending or deformation to which it is subjected in fabrication.

In carrying out my invention, the article, such as an oil still tube,automobile exhaust piping; superheater tube, or the like, which may bemade of the usual plain low carbon steel, is first subjected to asurface aluminizing treatment, preferably by the so-called calorizingtreatment, in which a thin surface alloyage of the aluminum with thesteel is formed. This is preferably carried out by the usual powdercalorizing process in which the tubes are placed in a retort with theusual calorizing batch containing about 89% aluminum oxide, 10% aluminumpowder and 1% of an energizer such as ammonium chloride, and heated at1600 to l800 Fahrenheit, preferably at about 1700 Fahrenheit, to cause asurface alloyage of the aluminum with the steel. Powder calorizing isdescribed in the Van Aller Patent 1,155,974 and the Gilson Patent1,091,057. Such calorizing treatment results in a; surface alloyage ofthe aluminum with the steel, forming a relatively thin surface alloyagelayer about .005 to .010 inches thick and containing an aluminumconcentration of from 50 to 75%.

The thickness of this aluminum alloyage may vary somewhat, dependingupon the time, temperature and percentage of aluminum in the batch, butis always a relatively thin surface layer of high aluminumconcentration.

After the calorizing has been carried out, there is a. fairly distinctline of demarcation between the aluminized surface layer and theunderlying steel, along which there is a tendency for the surface layerto spall off from the steel if the article is subjected to deformation.The surface alloyage having such high aluminum concentration possessespractically no ductility but is exceedingly brittle and hard.

The articles are next subjected to a prolonged heat treatment in aclosed retort at a temperature of about 1650 to 2000 Fahrenheit,preferably at about 1800 Fahrenheit. The heat treatment is sufficientlyprolonged so that the high aluminum concentration of the thin surfacealloyage is reduced to the point where the alloyage becomes sufficientlyductile to permit the required deformation of the article and where thealuminum is diffused into the steel with a relatively deep penetration,resulting in a tightly adherent aluminized surface. The line ofdemarcation is entirely obliterated and the aluminum concentrationtapers off gradually from the surface into the body of the steel,leaving no plane of weakness or exfoliation. The time for such heattreatment depends upon the temperature employed and the initial amountof aluminum applied to the surface by the calorizing treatment; Ingeneral, the time may vary fromabout 24 hours at 1800 Fahrenheit for thediffusion of a light calorized coating, up to 48 hours at 1800Fahrenheit for a heavier calorized coating.

In Figures 1 and 2 there is illustrated diagrammatically approximatelywhat is believed to take place. In Figure 1 the cross-hatched rectangleA B C D represents a fairly heavy calorizing as imparted by the initialpowder calorizing treatment and is illustrated as a surface alloyage ofabout 60% aluminum with a thickness of about .01 inches. There is afairly sharp line of demarcation between the aluminized surface and theparent metal as indicated by the line C D. A heat treatment for about 48hours at 1800 Fahrenheit results in a redistribution of the aluminumapproximately as indicated by the cross-hatched triangle A E F. Thepercentage of aluminum at the surface is reduced, as diagrammaticallyindicated, to about 30%, the percentage aluminum concentration taperingoff into the body of the parent steel roughly in proportion to thedistance of penetration. As illustrated, the total penetration is about0.04 inches, which'is relatively deep compared to the penetrationsecured by the ordinary commercial calorizing methods. In Figure 2 Ihave illustrated the operation in producing a somewhat lower surfacealuminum concentration. In this case the rectangle A B C D representsthe initial surface aluminization corresponding to a rather light powdercalorizing treatment and illustrated as about .003 inches thickness andhaving the usual aluminum concentration of about 60%. The heat treatmentfor about 24 hours at 1800 Fahrenheit results in causing a relativelydeep penetration of .025 inches of aluminum into the body of the steel,as illustrated by the cross-hatched triangle A E F.

Figures 1 and 2 illustrate diagrammatically typical instances. The exactpercentage of aluminum concentration at the surface and character ofpenetration may be varied to suit the individual requirements, as I willnow explain.

Iron-aluminum alloyages such as those universally produced by the usualcalorizing processes and containing over about 35% aluminum are verybrittle and hard and cannot be used on articles which requiredeformation or are subject to abrasion. However, if the aluminumconcentration be reduced below about 35%, the character of the alloyageundergoes a marked change in that the surface alloyage. becomes ductileand can be bent with the article without cracking. An aluminumconcentration of about 35% appears to be about the upper limit belowwhich a surface alloyage possesses some ductility. The lower limit ofaluminum concentration is determined by the required resistance of thesurface to oxidation or corrosion. If the oxidation and corrosiveconditions are not extremely severe, a surface of fair resistance may bemade with the aluminized surface diffused down to a surfaceconcentration of about 5%. However, 9% appears to be about the lowerlimit of aluminum concentration for very effective resistance tooxidation and corrosive attack.

In general, the greater the aluminum concentration, the higher thesurface stability, and balanced against this, in general, the lower thealuminum concentration, the greater the ductility. In the making of anyspecific article, the aluminum concentration is maintained as high aspossible while securing the requisite ductility. For example, in makingoil still tubes, which are subjected to fairly severe oxidation andcorrosive attack and in which extreme deformation is not required inexpanding the tubes in the end fittings, a surface aluminumconcentration of about 30% is preferred. In the making of automobileexhaust piping, in

' surface.

steel.

creased strength at high temperatures and.

' steel.

which the oxidizing and corrosive conditions are not as severe but inwhich the conditions of deformation and vibration are more severe, analuminum concentration of about 10 to 15% is preferred.

I will. now describe certain typical embodiments of my invention withreference to the manufacture of oil still tubes and superheater tubes,and of automobile exhaust piping, particularly as such description willset forth certain other advantageous effects of the heat treatment uponthe physical properties of the steel.

Oil still tubes In making oil still tubes, they are first given a fairlyheavy initial surface alloyage by the usual calorizing treatment,forming a surface alloyage layer of about .005 to .010 inches thickhaving the usual aluminum concentration of from about 50 to 75%. Thetubes are then subjected to a prolonged heat treatment, preferably about48 hours, at about 1800 Fahrenheit. This results in a deeply diffusedalloyage having about 25 to 30% aluminum concentration at the surfaceand tapering off to a depth of about .04 inches into the steel. I havefound that an aluminum alloyage containing at its surface from about 9to 35% aluminum concentration has the requisite resistance to oxidationagainst the heating gases on the outside of the tubes and against thecorrosive sulphurous compounds to which the inside of the tubes issubjected. Therefore, the heat treatment should be so regulated that thealloyage at the surface should not be over about 35% aluminumconcentration or below about 9% aluminum concentration. The higher thetemperature and the longer the time of heat treatment, the greater isthe diffusion of the aluminum into the steel with consequent deeperpenetration and decreased aluminum concentration at the I prefer toregulate the heat treatment so as to leave a percentage aluminumconcentration at the surface of about or to or usually about 25 to 30%,since this gives excellent resistance to oxidation and corrosioncombined with sufiicient ductility to withstand deformation in expandingthe ends of the tubes into their end fittings.

The heat treatment has another advantageous function in increasing theresistance of the steel against high temperature creep under tension.The prolonged heat treatment results in a grain growth of thecrystalline structure of the steel, which has been found to considerablyincrease the resistance of the steel to slow deformation or creep at thehigh temperatures of the oil still under the pressures maintained in thetubes.

The tubing most generally employed for oil still tubes at the presenttime is seamless tubing made of low carbon (about .08 to .18%)

I have found that a tubing having inwhich can still be successfullytreated in accordance with my process, can be made from a steelcontaining a small amount of molybdenum, about .25 to 2.50%, preferablyabout .50 to 1%. The carbon is preferably low, about .08 to .l8%. I havefound that the molybdenum used in such percentages does not interferewith the distribution and penetration of the aluminum into theAdditional strength at high temperatures may be secured by using a smallpercentage of chromium, about .25 to 2.50%, preferably about 1 to 1.50%,in conjunction with the molybdenum.

In Figure 5 there is illustrated a cross-section of an oil still tubetreated in accordance with my process, in which the tube is indicatedgenerally by reference numeral 1. The aluminum alloyage on the outsideof the tubes is indicated at 2, and the aluminum alloyage on the insideat 3. The steel between them is indicated at 4.

In Figure 3 there is illustrated the ends of two oil still tubes 1having their ends expanded into a standard end return bend fittingindicated generally by reference numeral 5. In applying the fitting tothe ends of the tubes 1, the ends of the tubes are put into the fittingand expanded by a tube expander in the usual way. As shown in thedrawings, the extreme end of a tube 1 is stretched or flared at 6. Also,near the end of the tube it is expanded as indicated at '7 to lock intoa recess in the internal bore of the fitting. The expanding of the tubesinto this type of fitting is carried out cold, and I have found that thesurface alloyage as produced by my process is sufficiently ductile andadherent to the underlying steel so that such cold expansion can takeplace without cracking or spalling 01f of the aluminized surface. Thecrystalline grain growth imparted by the heat treatment employed, while'it somewhat increases the stifiness of the metal cold as well asincreasing its resistance to high temperature creep, does not reduce thecold ductility enough to prevent expansion of. the ends of the tube:cold with the usual tube expander.

In Figure 4 I have illustrated the ends of the oil still tubes 1 as heldin another standard type of return bend end fitting. In this type offitting the end of the tube 1 is upset hot to form a flange l0-which isreceived in a recess in the internal bore of the fitting indicatedgenerally-by reference numeral 11. This requires a considerabledeformation of the end metal of the tube, but I have found that thesurface alloyage as produced in accordance with my process hassufilcient ductility to' resist cracking off or spalling during theupsetting operation and to resist cracking except minute surface crackswhich do not extend through the surface alloyage.

The oil still tubes processed in accordance with my treatment meet thesevere conditions required in the following respects:

1. The surface alloyage, because of its reduced aluminum concentrationat the surface below about 35%, and because of therelatively deeppenetration with a tapering off of the aluminum concentration into thebody of the parent metal, does not crack or spall off when the ends ofthe tubes are expanded cold into the end fittings, or when the ends ofthe tubes are upset hot for reception into the end fittings. An oilstill tube having the aluminized surface imparted by the usualcalorizing treatment cannot stand such deformation because the highconcentration surface alloyage is so brittle as to crack and spall off,such spalling occurring along the plane of relatively sharp demarcationbetween the high concentration alloyage and the steel beneath.

2. The alloyage having the surface concentration of from about 9 to 35%aluminum, still contains sufficient aluminum to effectively resist theoxidation at high temperatures to which the outsides of the tubes aresubjected with the hot gases of combustion employed to heat the still.

3. The aluminum concentration at the interior surface (of from about 9to 35%) is sufficient to effectively withstand the corrosive action ofthe sulphurous compounds frequentlyoccurring in petroleum. The corrosiveattack of the petroleum is not as severe as the oxidation to which theoutsides of the tubes are subjected, so that the interior aluminumalloyage steel resists such corrosive attack even if its richer surfaceportion is removed in the tube cleaning operations.

4. The surface alloyage on the interior of the tubes, while not brittleand strongly adherent to the parent metal, possesses relatively greathardness and resistance to abrasion, which is an important feature sincethe tubes must be frequently cleaned by passing through them tubecleaners having cutting blades for removing the tightly adherent carbondeposits. While the high aluminum concentration surface alloyageimparted by the.,,usual commercial calorizing process is extremely hard,it is, nevertheless, very brittle and tends to spall off when subjectedto the action of the tube cleaner, so that such tubes cannotsuccessfully withstand cleaning with the tube cleaners. When the stilltubes are made of the ordinary low carbon steel without the aluminizedsurface protection, the steel surface is so soft that in time it is cutaway and abradedby the tube cleaners. As a specific example of suchadvantage, I may cite a test in which a number of tubes treated inaccordance with my process 'were compared with a number of tubes of thesame low carbon steel untreated. A standard tube cleaner was passedthrough each tube for 150 passes at a speed of five feetper minute,which represents approximately the amount of tube cleaning required forfour years service of an oil still. Such tube cleaning resulted in nomeasurable abrasion of the surface formed by my process, but after suchtube cleaning the surface showed a'high polish. The plain steel tubesunder the same conditions lost 1/32nd of an inch due to abrasion by thetube cleaner, and the surface also showed bad scoring and scratching.

5. The heat treatment employed in my process for reducing the highsurface aluminum concentration and for deeply diffusing the aluminuminto the parent metal, considerably increases the resistance of thesteel against the slow deformation or creep at the high temperaturesemployed in the oil stills. Microscopic examination of the metal beforeand after my heat treatment indicates a considerable grain growth in thecrystalline structure of the steel, which is believed to be the cause ofsuch increased resistance to high temperature creep.

Metals at high temperatures do not possess a true elastic limit. If aspecimen is heated and immediately pulled in the manner employed forconducting room temperature tests on'steel, an apparent elastic limit isrecorded. However, if the steel in a heated condition is subjected toloads for an extended period of time, it is found that there is aconstant creep or flow of the metal which possesses the generalcharacteristics of a plastic material. This is the condition encountereddue to continued tension on the tube walls which are subjected to thestill pressure at the high still temperatures. A comparative test on amild steel tubing untreated, and the same tubing treated in accordancewith my process, showed that a load of 1400 pounds per square inch gavea creep value of 1% after 10,000 hours at 1100 Fahrenheit for theuntreated steel, whereas it required 2050 pounds per square inch to give1% creep value after 10,000 hours at 1100 Fahrenheit with the tubingsteel after treatment by my process. This shows a 46% increase in thehigh temperature strength of the steel. A lighter tubing can thereforebe employed if treated in accordance with my process, with consequentincrease in inside diameter and increased capacity of the still, as wellas greater heat conductivity through the thinner wall.

Incidentally, it may be remarked that the tubing subjected to the usualcalorizing process does not have such increased resistance to hightemperature creep as is imparted by the prolonged heat treatmentemployed in my process for the redistribution and diffusion of thecoating as applied by the usual calorizing' processes.

superheater tubes superheater tubes are employed in steam boilers forhighly superheating'the steam. The superheater tubes may be subjected towall temperatures of about 1300 to 1400 Fahrenheit and to high steampressures. The outsides of the tubes are subjected to the oxidizing andcorrosive attack of the furnace gases, while the insides of the tubesare subjected to oxidation due to the dissociation of the steam.

In Figure 9 is illustrated a typical superheater tube 30 showing thebending to which the tubing is subjected. While the bending is fairlysevere, it is generally carried out hot, so that aluminum concentrationssubstantially the same as those employed in oil still tubes may be used.superheater tubes are manufactured with substantially the same processconditions as described in connection with the oil still'tubes,

which do not need to be here repeated. As in the case of oil stilltubes, the prolonged heat treat-' ment results in an increase in thecreep resistance of the superheater tubes against steam pressures athigh temperatures. The molybdenum and molybdenum-chromium alloy steelsdescribed in connection with the oil still tubes may be advantageouslyemployed for superheater tubes.

The development of superheaters at the higher temperature ranges hasbeen retarded for lack of a moderately priced tube which willeffectively withstand the conditions of service from a standpoint ofoxidation, corrosion and creep imposed by such installations.

The molybdenum and chrome-molybdenum steels possess the requisite hightemperature strength, but do not have sufficient resistance to oxidationand corrosion, which properties are imparted by the surface aluminizing.On the other hand, surface aluminizing as applied by the usualcalorizing processes is not suitable because it will not withstand thedeformation in the fabrication of the tubes. My superheater tubes can bemade at a relatively low cost and at the same time possess all of therequisite properties required in superheater use.

Automobile exhaust piping Automobile exhaust piping is subject to bothhigh temperature oxidation and. corrosive attack. The pipe between theengine and muffler may become heated by exhaust gases up to 1200 to 1300Fahrenheit, particularly when it is lagged to protect floor boards, andis therefore subject to high temperature oxidation. The inside of thetube is also subjected to the corrosive attack of the hot gases. Thetail pipe beyond the mufiier is subjected to the corrosive attack of thecondensed gases, particularly in automobiles which are frequentlystarted and stopped. Plain steel tubing, which is usually employed, isfrequently perforated before the car has run 10,000 miles. Automobileexhaust piping is bent cold and is therefore subjected to severeconditions of deformation. In making automobile exhaust piping I preferto employ a surface aluminum concentration somewhat lower than thatrequired from oil still tubes and superheater tubes, since the oxidizingand corrosive conditions are not as severe, and because the loweraluminum concentration better meets the more severe deformation.

In' making automobile exhaust piping, ordinary low carbon (.08 to 18%)mild steel tubes of about 14 gauge is preferably employed. The lengthsof unbent tubing are first subjected to a rather light surfacealuminizing treatment, preferably by the usual powder calorizing method,which results in a rather thin surface alloyage, preferably about .003to .005 inches thick. The tubing is then subjected to a prolonged heattreatment in a closed retort, preferably for about 24 hours, at about1800 Fahrenheit. This results in reducing the concentration of aluminumat the surface and a diffusion or penetration of the aluminum relativelydeeply into the body of the steel, resulting in a highly ductile,tightly adherent, aluminized surface of suflicient concentration toresist the oxidizing and corrosive conditions encountered. Preferably,the heat treatment is carried out to produce a concentration at thesurface of between and 20% aluminum, since this combines good ductilitywith excellent resistance to the oxidation and acid corrosionencountered in use, although as in the case of the oil still tubes, thesurface concentration may vary from 9 to 35%. Good resistance, however,can be secured even if the surface concentration is reduced to as low asabout 5%.

The heat treatment required to produce the reduction in the surfacealuminum concentration and to cause the penetration of the aluminum intothe metal, when carried out as above described, results in coarseningthe granular structure of the steel which tends to make the piping morebrittle and less adaptable for the cold bending operations. Therefore,after the heat treatment described above, the tubing is subjected to arelatively long soaking period in the order of about 12 hours at thelower critical point, which, for mild steel tubing of, .08 to .18%carbon content, is about 1650 Fahrenheit. The tubing should then becooled rather rapidly in air to inhibit any tendency to a reversion ofthe structure to a coarse grain size. I have found that such treatmentrestores the ductility of the tubing so that it can be readily given thenecessary bending in fabricating the automobile exhaust piping system.

The tubing treated in accordance with the above described process is aconsiderable improvement over the plain tubing now employed. My tubinghas the requisite ductility for cold bending. The reduction in thealuminum concentration at the surface results in imparting to thealuminized surface suflicient ductility so that the tubing can be bentwithout surface cracking, and the diffusion of the aluminum into thesteel with the tapered off concentration overcomes any tendency for thecoating to become exfoliated along a cleavage plane, as would be thecase with the usual calorized coating. I have found that while thesurface concentration of aluminum is reduced greatly below that of thestandard calorized surface, in order to get the requisite ductility, thealuminum concentration is still sufficient to effectively resistoxidation and corrosive gaseous attack at high temperatures, as well asthe low temperature attack of acid condensate, to which the parts of theexhaust system are subjected in an automobile.

In Figure 5 of the drawings is illustrated a cross-section of thepiping, which is indicated generally by reference numeral 40. In thisfigure, reference numeral 41 indicates the outer aluminized surface and42 the inner aluminized surface, whichare separated by the body or core43 of the nonealurninized steel. The usual exhaust tubing is of 18 gaugethickness. I prefer to use somewhat heavier gauge, preferably about 14gauge, so as to leave the core of soft mild steel forming a substantialpercentage of I the cross-sectional area.

In Figure 3 there is illustrated the piece of tubing 44 which extendsfrom the engine exhaust manifold to the muiiier, the form illustratedbeing that of one of the commercial automobiles. jected to rather sharpbending requiring considerable ductility. This piece of tubing also maybe lagged with asbestos, which subjects it to a high temperature underthe oxidizing and corrosive gas attack conditions. In Figure 4 there isillustrated the tail pipe 45 of an automobile, illustrating the amountof bending to which such piping must be subjected.

While the present process has been developed primarily for the treatmentof the pieces of tubing constituting the tail pipe and the connectionbetween the exhaust manifold and the mufller,-the process can be appliedto other parts of the automobile exhaust piping system including themuflier and the exhaust manifold, since these parts are also subject tothe high temperature oxidation and corrosive gas attack, and I thereforeuse the words automobile exhaust piping as a term of general descriptionand not of limitation, and as applying to any of the parts of theexhaust system.

While I have above described the preferred process of treatingthe'automobile exhaust material, there are other specific methods whichmay be employed according to my process. One of these methods is toconduct the calorizing and subsequent heat treating operation at atemperature below the lower critical point of the steel, so that graingrowth with the impairment of ductility does not take place. For mildsteel tubing (.08 to .18% carbon)' commonly employed, the lower criticalpoint is about l650 Fahrenheit, so that in this specific method thecalorizing and heat treatment should both be conducted at, say, about1600 Fahrenheit. This method does away with the necessity of soaking atthe lower critical point to restore the grain structure. The calorizingand heat treating at the temperature of about 1600 Fahrenheit result ina rather light aluminum deposit and light As shown here, this tubing issubduced is usually adequate for the service conditions encountered.

Another method is to completely bend and fabricate the material beforecalorizing it. In

this case the powder calorizing is performed by the pack method, inwhich the work is packed in a sealed container with the usual calorizingbatch and subjected to a prolonged heat treatment. The alloying of thealuminum with the surface of the steel and subsequent diffusion takesplace in a single operation. The surface alloyage is usually light,since only that aluminum in contact with the surface of the steel isavailable for reaction. The pack method may be practiced on automobileexhaust material in one of two ways:

(a) The process may be carried out below the lower critical point of thesteel for a relatively long period, say, 24 to 48 hours, in which casethere is no grain growth of the steel and no need of subsequent grainrefinement.

(b) The time may be substantially shortened, say, 8 to 16 hours, byraising the temperature to about 1800 Fahrenheit, in which event it isnecessary to follow with soaking at the lower critical point to refinethe grain.

Other products The invention may be embodied in other specific articleswhich are intended for use at temperatures normally not in excess ofabout 1500 Fahrenheit. The specific articles described above, namely,oil still tubes, superheater tubes and automobile exhaust piping, are

not subjected to temperatures in excess of about 1500 Fahrenheit. Fortemperatures below about 1500 Fahrenheit, the diffusion and distributionof the aluminum secured by my heat treatment remains stable and thesurface is permanently protected against oxidation or corrosion. n theother hand, if the articles were to be subjected in use to temperaturesmuch in excessof about 1500 Fahrenheit, such higher temperatures wouldtend to dissipate the aluminum and reduce the surface concentration to apoint where it would no longer effectively resist oxidation andcorrosion.

The invention is particularly applicable to articles which are subjectto deformation in fabrication or in use, or subjected to abrasion ormechanical vibration. Examples of such articles are recuperator or airheater tubes, carbon black burner pipes, coke still bottoms, and boltingmaterials, particularly for use under heated oxidizing, and corrosiveconditions. These articles are preferably given a fairly heavy initialcalorizing treatment followed by the heat treatment described inconnection with the manufacture of oil still tubes, since this giveshigh resistance to oxidation and corrosion, combined with sufficientductility and adherence of the coating to meet the conditionsencountered.

Heat exchanger and condenser shells, and heat exchanger and condensertubes, are other examples of articles embodying my invention. Since theheat and corrosive conditions are not as severe, and since the steel isusually subjected to rather severe deformation in fabrication, I preferto employ a lower aluminum surface concentration as secured, forexample, by the process described in connection with the manufacture ofautomobile exhaust piping.

This application is a continuation-in-part of my copending applicationsSerial Nos. 684,922 and 684,923, both filed August 12, 1933.

While I have described certain specific embodiments of my invention, itis to be understood that the invention is not so limited, but

may be otherwise embodied and practiced within the scope of thefollowing claims.

I claim:

1. A steel article for use at temperatures normally not in excess ofabout 1500 Fahrenheit, having an aluminized surface with a surfaceconcentration of about to 35% aluminum tapering off gradually into thebody of the steel and characterized by its ductility and adherence andits high surface stability.

2. A steel article subject to deformation or abrasion and intended foruse at a temperature normally not in excess of about 1500 Fahrenheit,having an aluminized surface with a surface concentration of about 5 to35% aluminum tapering off gradually into the body "of the steel andcharacterized by its ductility and adherence and by its resistance tooxidation or corrosion.

3. A steel article subject to deformation or abrasion and intended foruse at a temperature normally not in excess of about 1500 Fahrenized bysufiicient ductility and adherence of such aluminized surfaces to permitexpansion of the tube into the usualend fittings and by resistance tooxidation at the outsidev of the tube and resistance to corrosion andtube cleaner abrasion at the inside of the tube.

5. A steel oil still tube having an aluminized surface with a surfaceconcentration of about 20 to 35% aluminum tapering off with relativelydeep penetration into thesteel and characterized by sumcient ductilityand adherence of such aluminized surface to permit expansion of the tubeinto the usual end fittings and by resistance to oxidation at theoutside of the tube and resistance to corrosion and tube cleanerabrasion at the inside of the tube.

6. A steel oil still tube having aluminized surfaces heat treated toproduce a surface concentration of about 9 to 35% aluminum withrelatively deep diffusion of the aluminum alloyage into the body of thesteel and characterized by the ductility and adherence of suchaluminized surfaces and their resistance to oxidation on the outside ofthe tube and their resistance to corrosion and tube cleaner abrasion onthe inside of the tube, and further characterized by an increasedresistance of the steel to creep under tension at high temperatures.

7. A steel oil still tube formed of an alloy steel containing from about.25 to 2.5% molybdenum and having aluminized surfaces with a surfaceconcentration of about 9 to 35% aluminum with relatively deeppenetration into the steel and characterized by sufllcient ductility andadherence of such aluminized surfaces to permit expansion of the tubeends into the usual fittings and by resistance of such surfaces tooxidation on the outside of the tube and by resistance to corrosion andtube cleaner abrasion on the inside of the tube, as well as beingcharacterized by high tensile strength at high temperatures.

8. An oil still tube formed of an alloy steel containing .25 to 2.5%molybdenum and from .25 to 2.5% chromium and having aluminized surfaceswith a surface concentration of about 9 to 35% aluminum with relativelydeep penetration into the steel and characterized by sufficientductility and adherence of such aluminized surfaces to permit expansionof the tube ends into the usual fittings and by resistance of suchsurfaces to oxidation on the outside of the tube and by resistance tocorrosion and tube cleaner abrasion on the inside of the tube, as wellas being characterized by high tensile strength at high temperatures.

9. Steel automobile exhaust pipinghaving an aluminized surface with asurface concentration of about to 35% aluminum tapering off graduallyinto the body of the steel and characterized by its ductility andadherence and by resistance to oxidation and corrosion to whichautomobile exhaust piping is subjected.

10. Steel automobile exhaust piping having an aluminized surface with asurface concentration of about to 20% aluminum tapering off graduallyinto the body of the steel and. characterized by its ductility andadherence and by resistance to oxidation and corrosion to whichautomobile exhaust piping is subjected.

11. An automobile exhaust system containing mild steel tubing having analuminized surface with a surface concentration of about 5 to"35%aluminum tapering off gradually into the body of the steel andcharacterized by suflicient ductility and adherence of such aluminizedsurface to permit bending of the tube in fabricating the exhaust systemand by its resistance to oxidation and corrosion encountered in suchsystem.

12. An automobile exhaust system containing mild steel tubing having analuminized surface with a surface concentration of about 10 to 20%aluminum tapering off gradually into the body of the steel andcharacterized by sufficient ductility and adherence of such aluminizedsurface to permit bending of the tube in fabricating the exhaust systemand by its resistance to oxidation and corrosion encountered in suchsystem. i

13. A superheater tube having an aluminized surface with a surfaceconcentration of about 9 to 35% aluminum tapering off with relativelydeep penetration into the steel and characterized by suflicieritductility and adherence of such aluminized surface to permit thedeformation encountered in fabricating the tube and. by its resistanceto oxidation and corrosion.

14. A superheater tube having an aluminized surface with a surfaceconcentration of about 20 to 35% aluminum tapering off with. relativelydeep penetration into the steel and characterized by sufficientductility and adherence of such aluminized surface to permit thedeformation encountered in fabricating the tube and by its resistance tooxidation and. corrosion.

15. A superheater tube having aluminized surfaces heat treated toproduce a surface concentration of about 9 to 35% aluminum withrelatively deep diffusion of the aluminum alloyage into the body of thesteel and characterized by the ductility and adherence of suchaluminized surfaces and their resistance to oxidation and corrosion, andfurther characterized by an increased resistance of the steel to creepunder tension at high temperatures.

16. A superheater tube formed of an alloy steel containing from about.25 to 2.5% molyL- denum and having aluminized surfaces with a surfaceconcentration of about 9 to 35% aluminum with relatively deeppenetration into the steel and. characterized by sufficient ductilityand adherence of such aluminized surfaces to permit the deformationencountered in fabricating the tube and by the resistance of suchsurfaces to oxidation and corrosion, as well as being characterized byhigh tensile strength at high temperatures.

1'1. A superheater tube formed of an alloy steel containing from about.25 to 2.5% molybdenum and from about .25 to 2.5% chromium, and havingaluminized surfaces with a surface concentration of about 9 to 35%aluminum with relatively deep penetration into the steel andcharacterized by suificient ductility and adherence of such aluminizedsurfaces to permit the deformation encountered in fabricating the tubeand by the resistance of such surfaces to oxidation and corrosion, aswell as being characterized by high tensile strength at hightemperatures.

BERTRAM J. SAYLES.

